core: Add Lock Content to undo history
@Reju noted in IRC that you can't undo a Lock Content toggle the way you can for the other lock options.
It turns out that it was intentionally dummied out in fb1a6725 (See comment here)
In 2.10, we don't create an Undo History entry for Lock Content. In 3.0, we do, but it doesn't actually do anything because of the commented out code. As all other Lock functions can be undone with Ctrl+Z, and every set_lock ()
function after Lock Content allowed this, I think it makes sense to fix this for Lock Content as well.